Further experiments on the control of corrosion of mild steel in crude oil distillation

Corrosion Science, Vol. 20, pp. 1195 to 1200 © P©rgamon Press Ltd. 1980. Printed in Great Britain.

0010.-938X/80/1201-1195 $02.00/0

F U R T H E R EXPERIMENTS ON THE CONTROL OF CORROSION OF MILD STEEL IN C R U D E OIL DISTILLATION* Y. AL-FARKH,F. H. AL-HAJJAR,H. S. HAMOUDand F. S. AL-SHAMALI Kuwait Institute for Scientific Research, P.O. Box 24885, Safat, Kuwait Abstraet--A still continuously refluxirtg kerosine and water with the addition of 3 ppm HCI and 800 ppm HsS was used to test the effect of 8 compounds upon the corrosion of mild steel over 1000 h. The organic inhibitors used included triazoles, thiones, hydrazides and a chlorochalcone. The most effective inhibitors over the temperature range investigated (50--180°C) were the hydrazides and the chlorochalcone. Other compounds were found to be reasonably effective at 50°C, but they became less effective as the temperature was increased.

INTRODUCTION

THE ECONOMIC control of the corrosion of crude oil distillation equipment such as columns and condensers continues to attract attention. We have been engaged in the preparation of a number of novel organic compounds suitable for use as corrosion inhibitors. For example, we have reported the synthesis of some organic nitrogen compounds of interest in this respect. Recently 8 compounds, including those previously reported, 1 have been prepared. Their inhibitor efficiency was evaluated by means of a technique previously described, ~ and which was found to give results over a 1000 h test period which were comparable to annual corrosion rates experienced in industry. The compounds concerned are named below: (.4) 5-phenyl 1,2,4 triazole-3-thione (B) 5-benzyl 1,2,4 triazole-3-thione (c) 5-~-naphthyl 1,2,4 triazole-3-thione (D) 4-aryl-6-p-chlorophenyl-5,6-dihydropyridimine-2(1 H)thione (E) 4-chlorophenyl-6-p-methoxyphenyl-5,6,-dihydropyrimidine-2-(l H)thione (F~) a-[3-(1,3-diphenyl-propan-l-one)]-~-(a-naphthoyl)hydrazine (at) ~-[(1,3-chlorophenyl-3-p-bromophenyl-propan-l-one)]-benzoyl-hydrazine (H) m-chlorochalcone. EXPERIMENTAL METHOD The steel previously described 1 was employed, and polished mild steel specimens exposed to a refluxing mixture of kerosine and water with suitable additions. At the outset of each experiment 3 ppm HC1 were added as N/100 solution, and H~S added daily from a cylinder at the rate of 800 ppm/d. Eight mild steel ring specimens polished to 800 emery paper and degreased were weighed and positioned at each of 5 positions in the column. The main temperatures at each position were, proceeding from the top, 50, 60, 85, 160 and 180°C ( ± 5 °) respectively. The steel analysis was C 0.150; Si 0.050; M n 0.80; S 0.037; P 0.010; Cu 0.018; Cr 0.030; M < 0.005; Fe. Each inhibitor was previously dissolved in ethanol to give a 0.01% w/v solution, and thereafter *Manuscript received 28 February 1979; in revised form 13 December 1979. fPreviously reported. I 1195

1196

Y. AL-FARKH et al.

added to the operating still at an appropriate daily rate. After 1000 h continuous operation the mild stool specimens were removed, descaled and weighed. General corrosion rates based on the total surface of the specimen were calculated, as were inhibitor officiencies from comparison with tim uninhibited corrosion rate. The latter were at successive stage positions from 1 to 5, 0.457, 0.508, 0.512, 0.736 and 0.838 mm.y -1 respectively.

Eight sp¢cimens were located at each column position (i.e. 40 in all); the corrosion rates at any position were av©raged, and any result departing by more than 20 70 from the mean was rejected. A new mean corrosion rate was then calculated to the nearest whole number. Such adjustments were not often required, and occurred randomly both as regards stage position and specimen order in the group, thus probably lacing due to inadequate specimen preparation. The unadjusted mean has also b ~ n indicated where appropriate in the results. EXPERIMENTAL

RESULTS

Figures 1-8 portray the results obtained with the compounds described in the introduction. It will be observed that although different structures have different effects there is an even more marked dependence upon concentration and temperature. The thiones showed their greatest effect at concentrations of 200 ppm (Figs. 1-3). In general such compounds also exhibited greatest effect at the lowest temperature (50°C) and the least at the highest temperature (180°C). The 5-benzyl compound was the exception to this behaviour as at 200 ppm its efficiency increased above 85 %. The corrosion products in the case of compound A were dark brown and fairly adherent, but in cases B and C they were much more friable. All products consisted largely of mixed oxides and sulphides of iron. In contrast both thiones (D and E) ex hibited a more regular behaviour (Figs. 4 and 5). They progressively reduced corrosion as their concentration was increased, but showed a marked tendency to fail at elevated temperatures; the corrosion products were dark brown and friable. Hydrazides (F and G) on the other hand proved to be much less sensitive to increased temperature, and compound G at 200 ppm was especially successful in this respect I00 --

80

60--

._U 40--

20--

I

o

50

I

I

[

(

I00

1,50

200

250

io

FIG. 1. Inhibitor .4. FIGS. I-8. Inhibitor etticiencies for mild stool specimens in refluxing kerosine-water 3 ppm HCI and 800 ppm H2S. Inhibitor concentration ×--50 ppm; e - - 1 0 0 ppm; + - - 1 5 0 ppm; A - - t o t a l moan value without adjustment.

Further experiments on the control of corrosion of mild steel

1197

I00 A

80+

+

60--

40--

20--

0

I

I

I

I

I

50

I00

150

200

250

FIo. 2.

Inhibitor B.

(Figs. 6-7). The corrosion scale in all cases was black and dense and difficult to remove. m-Chlorochalcone was found to be ineffective at 50 or 100 p p m but much better at 150-200 p p m (Fig. 8), when the effect of temperature was slight. The corrosion products were dark brown and dense, and often bore traces of elemental sulphur on the surface.

IOO,

80

o~ :>,

60 .u

==

+ x

+

40 .0 J~ c

20x

50

10o

15o t°

FIG,

3.

Inhibitor C.

I

I

200

250

1198

Y. AL-FARKH et al.

100

8O

=¢

"~

60

2o

I 0

50

I00

150

I

/

200

250

t °

Fro. 4. DISCUSSION

Inhibitor D. AND

CONCLUSIONS

It would appear that the test conditions in which one forms a condensate of immiscible kerosine and water contaminated with both HCI and HzS are ones that have a marked influence upon the behaviour of inhibitors. (I) The triazoles are capable of developing a fair degree of corrosion inhibition at the lower (or column-top) temperatures. However, at higher temperatures (which are encountered at progressively lower column positions) they tend to be less effective. Presumably the complex formed between the inhibitor and the metal or 100 --

80--

o~ 60--

.u_

O

40--

20

--

I

I

I

I

I

50

IQO

150

200

250

to

FIG. 5.

Inhibitor E.

Further experiments on the control of corrosion of mild steel

1199

I00

80

60 (IJ

40

20

I

50

0

I

1

I00

150

I 200

I

250

t*

FIG. 6. Inhibitor F. corrosion product was destabilized. An exception to this behaviour was 5-benzyl 1,2,4 triazole which at 200 ppm maintained the efficiency over the temperature range. (2) The thione structure was found to provide reasonable protection at the lowest temperature, i.e. when the acidic aqueous phase was predominant, but this behaviour was sharply impaired at higher temperatures. In common with many sulphur-containing organic compounds, decomposition in the oil phase probably occurs, thus tending to provide a more corrosive condition. 100

-

8O

g

so

"5

i

_~ ao

20

0

I

50

I

t

I00

150 t*

FIG. 7. Inhibitor G.

I

200

I

250

Y. AL-FARKH et al.

1200 I00

80

60

_~ 4o

5S 5S c

20

x

• _ _ x

I

I

I

I

I

50

I00

150

200

250

I*

FIG. 8.

Inhibitor H.

(3) By contrast, the hydrazides tested gave a fairly constant degree of protection over the temperature range. This behaviour compares with that shown by ammonia1 and at sufficiently high concentrations ( > 200 ppm) they should be successful inlaibitors in this application. (4) The m-chlorochalcone returned interesting results in that at the highest concentration employed, the inhibition was c a . 9 0 ~ over the temperature range. The appearance of elemental sulphur at the specimen surface would appear to suggest that a reaction occurs between the m-chlorochalcone and H~S resulting in the elimination of the latter. It is interesting that the interim testing of inhibitor samples by this method is able to indicate the more promising organic structures for use in the corroded sections of crude oil distillation units, namely the naphtha and kerosine sections of the column. We are continuing with the evaluation of the compounds described, and also to synthesize new substances of appropriate structure. Acknowledgements--Our thanks are due to the Director General of the Kuwait Institute for Scientific Research for permission to publish this work, and to Professor T. K. Ross for assistance with the practical work. REFERENCES 1. Y. A. AL-FARKH, F. H. AL-HAJJAR, F. S. AL-SrtAMALIand H. S. HAMOUD, Chem. Pharm. Bull. (Tokyo) 27, 264 (1979). 2. T. K. Ross and R. laEDRAM, Cortes. Sci. 17, 849 (1977).